It may be a simple thing to do this, I have been using firefox downloadhelper for a long time, but it’s limited just to single video download, then I stumble upon videograbber that offers an easy “Grab” button, and this is what I finally settled on.

Besides, I found its pro version really does the trick cause I figured out how to download videos in batch mode, and also the choice to convert video between various formats. Even more important, it looks easy to combine those clips. Specific say, while converting, simply check Merge videos into one, you will easily make your favorite videos for mixes.